Method and apparatus for determining the oxygen demand of oxidizable materials

ABSTRACT

METHOD AND APPARATUS FOR DETERMINING THE OXYGEN DEMAND OF A MATERIAL CONTAINING OXIDIZABLE COMPONENTS, WHICH METHOD INVOLVES THE COMBUSTION OF A SMALL SAMPLE OF THE MATERIAL TO BE ANALYZED IN A HEATED, CONTINUOUS STREAM OF CARBON DIOXIDE. CARBON MONOXIDE PRODUCED AS THE RESULT OF COMBUSTION WITH CARBON DIOXIDE RELATES DIRECTLY TO THE TOTAL OXYGEN DEMAND (TOD) OF THE SAMPLE. HENCE, QUANTITATIVE ANALYSIS OF THE COMBUSTION GASES FOR CARBON MONOXIDE YIELDS A MEASURE OF THE OXYGEN DEMAND OF THE SAMPLE. BY TOD IS MEANT THE NET OXYGEN DEMAND OF THE SAMPLE. THUS OXYGEN DISSOLVED IN THE SAMPLE AND OTHER OXIDANT SOURCE MATERIALS CONTAINED IN THE SAMPLE LOWER THE OXYGEN DEMAND OF THE SAMPLE AS IT IS MEASURED IN ACCORDANCE WITH THE INVENTION. THE CARBON DIOXIDE FEED GAS PLUS THE SAMPLE ARE INTRODUCED INTO A FIRST COMBUSTION TUBE, MOISTURE IS THEN REMOVED FROM THE EFFLUENT, AND THE MOISTURE-REDUCED EFFLUENT THEN IS PASSED THROUGH A SECOND COMBUSTION TUBE AND THENCE THROUGH A SUITABLE CARBON MONOXIDE DETECTOR.

V. A. STENGER March 2; 1971 3,567 ;386 I v METHOD AND APPARATUS FORDETERMINING THE OXYG 2 sheets-sheen DEMAND OF OXIDIZABLE MATERIALSQFiledFb} 1969 Oxyyen o mana boy oer //'/er March 2, 1971 Filed Feb; 5,1969 Raw sewage Je/l/ed 2hr:

/ 5 9 l2 Scamp/e number:

V. A. STENGER METHOD AND APPARATUS FOR DETERMINING THE OXYGEN DEMAND OFOXIDIZABLE MATERIALS Oxygen o ema qj myper/f/er 2 Sheets-Sheet 2 Rawsewage free/ed and seH/ea 2/u-s.

/ 5, 9 12 Samp/e numbers INVENTOR. Vern on 4. S/enyer HGENT,

United States Patent O 3,567,386 METHOD AND APPARATUS FOR DETERMIN- INGTHE OXYGEN DEMAND F OXIDIZA- BLE MATERIALS Vernon A. Stenger, Midland,Mich., assignor to The Dow Chemical Company, Midland, Mich.Continuation-impart of application Ser. No. 710,339, Mar. 4, 1968. Thisapplication Feb. 3, 1969, Ser. No. 801,231

Int. Cl. G01n 31/10, 3]/12, 33/16 US. Cl. 23-430 18 Claims ABSTRACT OFTHE DISCLOSURE Method and apparatus for determining the oxygen demand ofa material containing oxidizable components, which method involves thecombustion of a small sample of the material to be analyzed in a heated,continuous stream of carbon dioxide. Carbon monoxide produced as theresult of combustion with carbon dioxide relates directly to the totaloxygen demand (TOD) of the sample. Hence, quantitative analysis of thecombustion gases for carbon monoxide yields a measure of the oxygendemand of the sample. By TOD is meant the net oxygen demand of thesample. Thus oxygen dissolved in the sample and other oxidant sourcematerials contained in the sample lower the oxygen demand of the sampleas it is measured in accordance with the invention.

The carbon dioxide feed gas plus the sample are introduced into a firstcombustion tube, moisture is then removed from the eflluent, and themoisture-reduced effluent then is passed through a second combustiontube and thence through a suitable carbon monoxide detector.

BACKGROUND OF THE INVENTION This is a continuation-in-part of Vernon A.Stengers application Ser. No. 710,339, now abandoned filed Mar. 4, 1968,for Method and Apparatus for Determining the Oxygen Demand of OxidizableMaterials.

The present invention relates to the problem of analyzing combustible oroxidizable materials for their oxygen demand. A special and most usefulembodiment of the invention involves determining the total oxygen demand(TOD) of aqueous systems, e.g., waste streams. The invention isparticularly concerned with a method and apparatus for measuring the TODof oxidizable materials.

In the field of sewage treatment, Chemical oxygen demand (COD) has longbeen used as a measure of pollution. A common method for determining CODis described in a publication of the American Public Health Association,Standard Methods for the Examination of Water and Waste Water, 11thEdition, New York, 1960, page 399. Basically, the technique describedtherein involves oxidation of the sewage sample with potassiumdichromate in 50 percent sulfuric acid. The amount of dichromate reactedreflects the extent of oxidation, thus titration of residual dichromateyields a measure of the oxygen demand of the system. Although thismethod yields useful results, the length of time to achieve an analysisis excessive.

It would be desirable, and it is a principal object of the presentinvention to provide a rapid, more accurate method for measuring theoxygen demand of an aqueous system. A further and more comprehensivepurpose of the invention is to provide a convenient method for measuringthe oxygen demand of any oxidizable material. An additional object is toprovide apparatus for carrying out the foregoing determinations. Stillfurther objects are to provide methods and apparatus which increase thesensitivity of the measurement. These objects, and other benefits, as

ice

will become apparent hereinafter, are accomplished in accordance withthe present invention.

Accordingly, the invention involves a method for determining the oxygendemand of a material containing oxidizable components, which methodinvolves the combustion of a small sample of the material to be analyzedin a heated, continuous stream of carbon dioxide. After initialcombustion of the sample, moisture is removed and a further combustionoperation is effected.

As will be shown in the following examples, the carbon monoxide producedas the result of combustion with carbon dioxide relates directly to thetotal oxygen demand (TOD) of the sample. Hence, quantitative analysis ofthe combustion gases for carbon monoxide yields a measure of the oxygendemand of the sample. As used herein, combustion refers to the reactionor equilibration of carbon dioxide with an oxidizable material in thesense that the oxidant (carbon dioxide) is reduced and the oxidizablematerial is oxidized. By TOD is meant the net oxyen demand of thesample. Thus, oxygen dissolved in the sample and other oxidant sourcematerials contained in the sample lower the oxygen demand of the sampleas it is measured in accordance with the invention.

In process terms, the invention involves establishing a flowing feed gasstream containing carbon dioxide as essentially the sole oxidant. Thisgas stream is passed through a combustion conduit having a heating zoneat a temperature high enough to cause some combustion, or equilibration,of the oxidizable components of the ma terial to be analyzed with carbondioxide. Such equilibration occurs to a degree at temperatures as low asabout 500 C., but preferred and more uniform combustion is achieved attemperatures above about 650 C. The upper end of the temperature rangeis limited by the fusion temperature of materials used in the heatedzone of the combustion conduit, but it is preferred that the temperatureshould not exceed 1000 C.

After passing through a first combustion conduit, moisture is removedand the gaseous material from the first conduit is passed through asecond, similar combustion conduit.

Contained within the heating zone of each combustion conduit is agase-permeable catalyst bed through which the carbon dioxide-containinggas stream flows. The bed is preferabl at least about 2 cm. in length.It is the function of the catalyst bed to promote the equilibration ofcarbon dioxide with oxidizable components of the material analyzed toproduce carbon monoxide.

Suitable catalyst materials include, for example, high melting noblemetals such as platinum, palladium, iridium, rhodium, ruthenium andgold. Siliceous materials such as quartz are effective to a degree. Apreferred catalyst is platinum. For purposes of economy, the noblemetals are used in a form which presents a large surface area per unitweight of the metal. Often such catalysts are coated on an inertsupport. To be suitable in general, a catalyst should be efiectivelyfree from substances that reduce carbon dioxide to carbon monoxide, orsupply oxygen to organic matter in the sample. Thus, iron, nickel,copper and similar metals, which are reactive either with carbondioxide, carbon monoxide, oxygen, or components of the samples, shouldbe excluded from the high temperature zone. Similarly, the higher oxidesof most elements should be excluded.

From the heating zone and catalyst bed of the second combustion tube,the ga stream is passed into an analyzer for quantitatively determiningthe amount of carbon monoxide in the presence of carbon dioxide.Analytical devices for this purpose are known. One particularly suitablefor use in the present invention is a non-dispersive infrared analyzerwhich produces an electrical signal in proportion to carbon monoxidecontent of the gas stream.

3 The signal may be read-out by any convenient means such as a graphicrecorder.

Having established a carbon-dioxide feed gas stream flowing through thecombustion conduit and thence into the carbon-monoxide analyzer, aquantity of the combustible material to be analyzed is inserted into theheated zone of the first combustion conduit on the upstream side of thecatalyst bed. The continued flow of the gas stream sweeps gaseousproduct formed through the catalyst bed. Eflluent gas from the heatedzone of the first combustion conduit containing the equilibrated gaseousreaction product of carbon dioxide and oxidizable component of thematerial to be analyzed flows through a moisture removal device, througha second combustion conduit, and then into the carbon-monoxide analyzerwhereby the incremental increase in carbon monoxide of the gas stream ismeasured.

In the preferred practice of the invention, this measurement is obtainedin the form of an electrical signal which is a function of thecarbon-monoxide content of the effluent gas. Such a signal is readilycalibrated to provide a direct reading of carbon monoxide produced and,as will be demonstrated hereinafter, the total oxygen demand (TOD) ofthe sample analyzed.

It will be seen that the feed gas serves simultaneously as the reagentwhich oxidizes reducing material in the sample, and as a carrier gaswhich sweeps the reaction products from the combustion zone and throughthe detector. After the gas has left the combustion conduit, it isreferred to as the effluent gas.

The feed gas containing carbon dioxide as essentially the sole oxidantmay also contain any one or more of the inert gases such as nitrogen,helium, argon, krypton and the like and ordinarily will contain somesmall amount of oxygen as an impurity.

Although a gas containing small amount of oxidizing components can beused in the practice of the invention to provide useful results, greateraccuracy and sensitivity is obtained by assuring effective eliminationof oxygen and other gases of greater oxidation potential than carbondioxide by incorporating a volatile reducing component into the ga in anamount more than suflicient to react with oxidizing impurities in thefeed gas. Any one of a number of reducing reagents such as hydrogen,carbon monoxide, methanol, acetone, or ammonia may be incorporated incontrolled amounts into the feed gas stream for this purpose. When thegas mixture is heated, the reducing component reacts with the oxidizingimpurity.

Best results are achieved in the following manner: Carbon dioxide or amixture thereof with an inert gas such as nitrogen or argon is passedthrough a heated, gaspermeable bed of carbon. By suitably adjusting thetemperature of the carbon bed, a small portion of the carbon dioxide isreduced to carbon monoxide. Temperatures of the carbon bed usually fallwithin the range from about 450 to 650 C. Within the combustion zone,this carbon monoxide reacts with any oxygen or other oxidizingimpurities present. The atmosphere thus produced is, for the purposes ofthe invention, actually an oxidizing atmosphere in which carbon dioxideis effectively the sole oxidant.

With a given flow rate and composition of carbon dioxide-containing gas,the reaction of the gas in a carbon bed at a given temperature producesa feed gas with uniform or constant concentration of carbon monoxide.The presence of carbon monoxide in the feed gas at a constant low levelrelative to the carbon dioxide therein does not interfere with themeasurement of the incremental increase of carbon monoxide resultingfrom the reduction of carbon dioxide by the sample to be analyzed.

In another mode of operation, the reducing quality of the feed gas at asomewhat higher carbon monoxide level afiords an opportunity to measurethe net oxidizing capacity (NOC) of materials analyzed rather than theirTOD. This occurs as the result of the generation of a negative signal.,Such a signal is defined by a dip in the carbon monoxide concentrationof the eflluent gas as the result of the oxidation of carbon monoxide byoxidizing components of the sample. Whether the same has an oxygendemand or an oxidizing capacity can be determined readilyby observingwhether the carbon monoxide-concentration of the effluent gas isincreased or decreased upon injection of the sample. When the inventionis used in this manner, enough carbon monoxide is introduced into thefeed gas to fully reduce the oxidizing capacity of the sample. Usually,the feed gas will contain at least about 0.05 volume percent carbonmonoxide. Normally, it will not exceed 1 volume percent carbon monoxide,but higher amounts may be used if desired.

The gaseous effiuent from the first heating zone is passed through acooling zone wherein the temperature of the gas stream is lowered to atemperature below that of the apparatus used for detecting the carbonmonoxide. Thus, moisture, if any is present, is largely separated fromthe gas stream, and then the gas stream is passed through a secondheating zone prior to its entry into the detector. Moisture condensateis accumulated in the cooling zone and means are provided for itscollection and removal as needed.

As previously stated, the invention is applicable to the analysis of allmaterials containing oxidizable components. Thus, gases and solids aswell as liquids can be analyzed for their TOD in accordance with theinvention. Sample size is not critical, but small samples on the orderof 0.001 to 0.1 cubic centimeter for liquid and solid samples and 0.001to 5 cubic centimeters for gas samples permit the use of equipment ofconvenient design.

A major and most useful application of the invention is to the analysisof aqueous systems containing oxidizable components. A special designconsideration important to the success of such analysis is thepositioning of the catalyst bed within the combustion conduits. For suchoperation, the catalyst bed is positioned within the heated zone of acombustion conduit at some distance from the gas inlet. This distance issufficient to define, in conjunction with the confines of the conduititself, a sample expansion zone within the heated zone. Upon injectionof the liquid sample to be analyzed, the sample generates for an instantsome back pressure, particularly in the first combustion conduit. Thefeed gas within the sample expansion ZOne at the instant of injectionforms a gas blanket which prevents significant diffusion or back fiow ofsample vapors out of the heated zone. Sufficient volume of the sampleexpansion zone is indicated by the absence of condensate formation, inthe first combustion conduit, in the inlet of the feed gas stream.

For best operation in analyzing liquid materials, the line of sampleinjection should deposit a sample at about, or on, the upstream face ofthe catalyst bed of the first combustion conduit. A line of injectionwhich is essentially parallel to the longitudinal axis of the combustionconduit and the application of sufficient injection force assures such aresult.

Various carbon monoxide detection techniques enable determination of thetotal amount of carbon monoxide formed according to the integral whereinQco is the amount of carbon monoxide generated upon injection of thetest sample and dq/dt is the differential of carbon monoxide in theeffluent gas at any one instant. The time period of carbon monoxidevariation from the feed gas stream is defined by t t The preferred modeof operation involves correlating carbon monoxide content with somecharacteristic of an electrical signal generated by the carbon monoxidedetector. For example, an amperometric or potentiometric signal willexhibit displacement from a normal base line. The height or amplitude ofthe displacement can be correlated with the change in carbon monoxidecontent of the efliuent gas. To achieve such operation, however, certainparameters of the process should be controlled to provide reproducibleresults. For instance, it is necessary that the feed gas stream becontrolled to a predetermined constant rate of flow (predetermined meanspreset level: knowledge of absolute flow rates is not necessary). Itwill be determined for any particular equipment design chosen, i.e.volume of combustion conduit, flow rate capacity of carbon monoxidedetector and temperature of the combustion conduit that there will be arange of flow rates over which optimum signals will be generated. Thus,for equipment of a given design, an optimum flow rate is readilydetermined by measuring a sample with a known TOD over a series ofincrementally increasing flow rates. In this manner, an optimum flowrate will be defined which produces a sharp and discriminating signaland is preferably relatively insensitive to minor variations in flowrate. This technique will be illustrated with reference to a particularapparatus in the examples.

Nonmally with equipment of convenient design the combustion conduit willhave a bed volume Within the range from about 10 to 200 cubiccentimeters, preferably from about 25 to about 75 cubic centimeters. Bybed volume is meant the total volume of the heated zone within thecombustion conduit. For liquid samples, the sample size will usuallyrange from about 0.005 to about 0.5 percent, preferably 0.01 to 0.1percent, of the bed volume. Combustion temperatures in the combustionconduits are usually within the range of about 800 to 900 C. Suchtemperatures promote efiicient equilibration of carbon dioxide withoxidizable components of the test sample and signal characteristicssubsequently generated by electrical detectors of carbon monoxide arerelatively independent of small variations in the combustiontemperature.

Detectors which may be used to measure the carbon monoxide in theefliuent gases from the combustion conduit include any of the knownmeans for quantitatively analyzing a gas stream for its carbon monoxidecontent. As previously mentioned, the preferred detector produces anelectrical signal, the strength of which can be correlated with theconcentration of the measured quantity. A preferred detector is anon-dispersive, infrared analyzer sensitized for carbon monoxide. Asignal output from such an analyzer is adapted by a suitable amplifierand graphic read-out means, such as a strip chart recorder, to providereadings which can be converted to, or read directly as carbon monoxideconcentration in the efiluent gas and hence the TOD of the test sample.To provide comparable analytical leaings for calibration of thegenerated signal, care must be exercised to insure that test samplevolumes, amplifier gain, recorder voltage, and process operatingparameters involving temperature and gas flow rates are identical orwithin operational levels at which the analytical results areindependent of these variables.

Apparatus for carrying out the described analytical process, and certainpreferred embodiments thereof are illustrated in the accompanyingdrawings.

FIG. 1 is a schematic drawing of a complete apparatus suitable foraccomplishing the analysis of liquid of gaseous materials containingoxidizable components. The figure also illustrates the optional carbonmonoxide generator.

FIG. 2 is a detailed illustration of a combustion conduit containing acatalyst bed.

FIGS. 3 and 4 show comparative results of COD measurements made by theprior art technique and the TOD measurement of the present invention ontwo series of sewage samples.

FIG. 5 shows an alternative feed/carrier gas source and flow controlmeans.

The apparatus of FIG. 1 comprises carbon dioxide feed gas supply means2, sample injection means 3,

heating means 4, combustion conduit 22 within the heating means 4,cooling means 5, condensate removal means 6 and drying means 32 integralwith the cooling means 5, combustion conduit 85 Within the heating means4, and carbon monoxide detection means 7. In the illustrated preferredembodiment, the feed gas supply means comprises a carbon dioxide gastank 11 which feeds carbon dioxide through a series arrangement of apressure regulator 12, turn-off valve 13, and flow meter 14. This seriesarrangement constitutes a gas flow control means 8.

From the gas flow control means 8 the feed gas stream enters a carbonmonoxide generator 9. This consists of a furnace 19 having a heatingzone 17 in which there is positioned a conduit 18. Within the conduit18, is a gas permeable carbon bed 20 held in place by gas-permeable,positioning elements 10, 21 at each end of the bed 20. Temperaturecontrol in the carbon monoxide generating means is obtained with thepower control 26. Temperature readings are obtained by means of apyrometer 27. Each end of the conduit 18 is provided with connectingmeans 16 for attachment with preceding and succeeding apparatuscomponents.

The feed gas stream flows from the carbon monoxide generator 9 through acheck valve 15 into the combustion heating means 4 comprising a furnace24 in which there is a combustion conduit 22 having a heating zone 25.Temperature control in the heating means is achieved through a powercontrol 30. The temperature is measured by means of a pyrometer 28. Atthe gas inlet end of the combustion tube 22 is an injection means 3,such as the illustrated syringe 23-.

Gaseous eflluent from the combustion conduit 22 passes into coolingmeans 5, which in the illustration is an aircooled condenser 29 equippedwith condensate removal means 6 in the form of a stopcock 31 fordischarge of accumulated condensate. The cooled effiuent gases also passthrough drying means 32 incorporated in cooling means 5 and then througha second combustion tube 85, similar to the tube 22, in the heatingmeans 4, then through a filter to remove any solid particulate matterand thence into carbon monoxide-detection means 7.

The illustrated carbon monoxide detection means 7 consists of anelectrically connected combination of a non-dispersive, carbonmonoxide-sensitized, infrared analyzer 35. This analyzer produces avariable voltage signal to be amplified by means of a low-voltageamplifier 38. The enhanced electrical signal is fed into a continuousgraphic recorder 39 which produces a curve on a paper strip 42. Eitherthe amplitude of or the area under the curve 41 is a function of thecarbon monoxide in the effluent gases measured in the detection cell 36of the infrared analyzer 35. Useful controls in the detection means arethe amplifier gain control 43 and the recording voltage range control44.

After passing through the detection cell 36, the efiluent gas may bedischarged to the atmosphere through a vent 37 or by means of a valve 33returned through optional gas-return means comprising a conduit 79 to agas blanket acket surrounding a portion of the injection syringe 23. Gasreturned in this manner provides a blanket of oxygen-free gas insulatingthe injection port to the combustion conduit.

The various components of the foregoing apparatus are mterconnected toprovide a continuous gas stream with suitable gas conveying conduits 69,70', 71, 72, 73, 74, 75, 76 and 78.

In FIG. 2, the combustion conduit 22 is shown in more detail. Itconsists of two separable parts which are a feed gas inlet 54 and acylindrical combustion tube '51. Seated within the feed gas inlet 54 isan injection tube 52 adapted to receive the syringe 23. The injectiontube 52 is aligned in a direction essentially parallel to thelongitudinal axis of the combustion tube 51 and contains a reduced innerdiameter part 91 between the jacket 80 and the inlet 58. The syringe 23,whose needle extends through the reduced diameter part 91, is surroundedby a protective gas jacket 80. The feed gas inlet 54 is coupled with thecylindrical combustion tube 51 through a ground glass joint 53. Withinthe cylindrical combustion tube 51 is a catalyst bed 57 of platinumgauze balls 61 maintained in place by means of catalyst-positioningelements 59 and 60, the latter of which is seated against an indentation63 in the tube 51. Each end of the assembled combustion conduit 22 isadapted for coupling with preceding and succeeding apparatus elements.The upstream feed gas inlet as a small tubular nipple 58 and thedownstream outlet, coupling means is a ball portion 62 of a ball joint.

The combustion conduit 85, of course, has no feed gas inlet 54, as such.

Certain preferred embodiments of the above-described fundamentalapparatus components have been set forth. Numerous alternatives willoccur to those skilled in the art. Modifications necesary to adapt theapparatus for the analysis of solid and gaseous samples, as well asliquid samples, will readily occur to those skilled in the art.

For instance, with regard to the feed gas supply means 2, it is onlynecessary that there be provided a confined stream of carbon dioxidepreferably subject to precise flow rate control. With respect topreferred Operation, knowledge of the actual flow rate is not necessaryso long as the gas flow rate can be controlled to a predetermined andconstant rate. To this end any combination of mechanical means forsupplying and regulating a gas stream can be used in place of thatillustrated. Insofar as heating means 4 is concerned, apparatus capableof providing controlled heating over a temperature range of 500 to 1000C. can be used. Although an electric resistance furnace is efiicient forthis purpose, induction heating a means, or any other convenient heatingmeans, can be used.

Similarly, sample injection means 3 can be provided by any mechanicalapparatus capable of supplying measured aliquots of materials andinserting them into the heating zone 25 of the combustion conduit 22.For example, direct insertion of a liquid sample to be analyzed into theheating zone 25 can be accomplished by sprayers adapted to providecontrolled amounts of sample spray. Injection of solid samples isreadily achieved by known means. For instance, if the combustion tube 22is aligned vertically, the sample is simply dropped into the heatedzone.

Cooling of gaseous effiuent from the conduit 22 can be accomplished inconventional manner such as by passing the gaseous efiluent through theillustrated aircooled condenser 29. Alternately, water-cooled condensersare effective for this purpose.

Although it is not necessary to operability, it is preferred in order toincrease instrument sensitivity to employ the dryer element 32 whichwill further reduce any moisture entrained in the gaseous product priorto its introduction into the second combustion tube 85.

The particular carbon monoxide detection means 7 described above ispreferred but any analytical apparatus capable of indicating thequantity of carbon monoxide in the gaseous product with desiredsensitivity and specificity can be used. Illustratively, fuel cells andgalvanic sensing devices may be adapted for the analysis of carbonmonoxide.

It has been found, particularly with some types of samples, that watervapor and carbon monoxide reacted to form hydrogen and carbon dioxide,and vice versa, in the combustion conduit. Since the reaction isreversible, cooling and drying the efiiuent from the conduit 22 toremove moisture and then passing the drier effluent through theconduction tube 85 cause the reaction to be driven towards the H O+COside, improving the accuracy of the analytical results over thoseusually obtainable when only a single combustion tube is used. Forexample, in analyzing for volatile materials such as the lower boilingalcohols, acetone, lower phenols and the like, the analytical accuracyis increased from about percent to about percent.

Materials of construction employed in the above combustion gas trainmust generally meet the criteria of having resistance to carbon dioxideand moisture. Moreover, it is desirable, at least in the gaseOus producttrain, that materials of construction be essentially nonreactive tocarbon monoxide. Within the combustion zone itself, it is necessary thatthe materials of construction be inert to the combustion products ofsamples analyzed at the ele vated temperatures used for combustion. Suchmaterials include, for example, fused silica, Vycor glass, glazedceramics and the like siliceous materials.

In a specific embodiment of the above-described apparatus shown in FIG.1, A inch stainless steel tubing was utilized to provide the connectingconduits 69, 70 and 71, and inch butyl rubber tubing was utilized toprovide the connecting conduits 72, 73, 74, 75, 76, 77, 78 and 79. Thecarbon dioxide pressure regulator 12 was a Watts Regulator Type 26 ModelM1 and the valve 13 consisted of a Hoke needle valve. The flow rate wasmeasured with a Brooks Flowmeter 14 Type 2-11l0 with an R-2-l5AA tubeand a stainless steel float.

The carbon generator 9 consisted of an electric muffie furnace 19operating on a voltage of volts and a maximum power consumption of 900-watts. The power control 26 was a Powerstat variable voltagetransformer. An Assembly Products, Inc. pyrometer 27 Model 4526 was usedto indicate the temperature.

A cylindrical tube 18 consisting of fused silica and having an insidediameter of 1.2! centimeters and a length of 40 centimeters was used inthe construction of the heated zone 17 of the carbon monoxide generator.Within the tube 18 at about 24 centimeters from the inlet end thereofwas placed a gas-permeable carbon bed 20 about 4 centimeters longconsisting of granulated cocoanut charcoal held in place by positioningelements 10 and 21 of quartz wool one centimeter long at each end. Eachend of the carbon monoxide generator tube was provided with a ball jointas coupling means 16. A Kimble valve No. 38006 was used for the checkvalve 15.

Combustion-supporting temperatures within the combustion conduits 22 and85 were generated with an electric mufile furnace 24 operating on avoltage of 120 volts and a maximum power consumption of 900 watts. Thepower control 30 was a Powerstat variable voltage transformer.

The combustion tube 51 was a fused silica cylinder having an insidediameter of 1.27 centimeters and a length of about 40 centimeters. Theheated zone 25 of the combustion conduit 22 was about 30 centimeterslong. A gas inlet 54 was provided in the form of a tubular glass T, withthe cross bar of the T having a Vycor ground glass joint 53 at one endfor coupling with the fused silica combustion tube 51 and a No. 18stainless steel syringe needle 93 about 4.8 centimeters long seated inthe opposite end of the cross bar as receiving means for sampleinjection means in the. form of a syringe. When the components of thecombustion conduit 22 were assembled, the needle 98 was directed in aline essentially parallel with the longitudinal axis of the combustiontube 51. The stem of the tubular glass T provided a nipple 58 forconnection with the A inch gum rubber interconnecting conduit 74. AHamilton No. 705N syringe 23 was employed as the injection means 3.

Within the combustion tube 51 at about 24 centimeters from the inlet.end thereof was placed a catalyst bed 57 about 7 centimeters longconstructed of platinum gauze balls 61. The catalyst. bed was held inplace by catalyst positioning elements 59 and 60 in the form of quartzwool plugs one centimeter long on both ends of the platinum gauze balls.The catalyst bed was formed by gently tamping one quartz wool plug intoplace against retaining indentation 63 within the combustion tube 51with a glass rod, adding the platinum gauze balls and then the secondquartz wool plug. After its component parts had been assembled, thecombustion conduit 22 was placed within the electric mufile furnace 24so that the tip of the syringe needle 93 was outside the heating zone ofthe furnace 24 but yet in position such that, upon injection of theaqueous sample, the full amount thereof was deposited within the heatingzone 25 of the combustion conduit 22.

The gaseous products produced upon injection of a test sample wereconducted through a gas train consisting of a series arrangement of anair-cooled condenser 5, a U-shaped water trap 29, combustion conduit 85,and a gas drying element 32 usually containing Drierite. The water trap29 was adapted for intermittent drainage of accumulated water by meansof a stopcock 31. The interconnecting conduit 76 consists of inch butylrubber tubing.

Carbon monoxide detection means 7 employed with the foregoing apparatusconsisted of an infrared analyzer 35 (Beckman Model 21) equipped with a13.3 centimeter detection cell 36 sensitized for determination of carbonmonoxide. The detection cell 36 was maintained at a temperature of 45 C.to prevent the formation of condensate which would interfere with theaccuracy of the analytical result. Output from the analyzer 35 was fedby electrical leads 64 and 65 to a low voltage amplifier 38.Subsequently, the amplified output of the analyzer was fed into agraphic recorder 39 (Sargent Model MR) through electrical leads 6'6 and67. The recorder 39 was set by the voltage recording range control 44 tooperate in the 2.5 millivolt range. The gain control 43 of the amplifier38 was set at a predetermined level to provide a desired response in therecorder 39.

EXAMPLE 1 The method of the invention has been applied to the analysisof waste waters. Two waste streams were tested. One stream was a rawsewage which had been clarified by settling for two hours. The otherstream was from the same raw sewage but had been treated with a highmolecular weight, anionic polymer flocculant and the resultingsuspension also settled for two hours. Twelve daily composite sampleswere taken from each stream.

Each of the composite samples was divided into two portions, one ofwhich was subjected to a conventional COD analysis by chemicaloxidation. The other was blended for minutes in a Waring Blendor toproduce a uniform dispersion, which was then subjected to analysis inaccordance with the invention.

The results of these experiments are plotted in FIGS. 3 and 4. A closecorrelation in the results of the two methods is evident.

EXAMPLE 2 The general applicability of the analytical process set forthherein is illustrated according to the following mathematical treatment.Assuming an oxidation reaction for the types of compounds most likely tooccur in domestic waste streams, one obtains a generalized equation asfollows:

Manifestly, to balance the above equation the value of n (the number ofoxygen atoms required) is:

Solving the equation for n requires the determination of threevariables. The value of a can be determined as the total carbonaccording to the method of patent application Ser. No. 380,597, now US.Pat. No. 3,246,435. However, the values of b and d are independentvariables not readily measured, particularly with a dilute aqueous 10sample. Thus, there exists neither a true correlation with total carbonnor a technique for the direct measurement of b and d in the aboveequation to provide a useful method of determining n, the oxygen demand.

In accordance with the invention, however, carbon di oxide gives up apart of its oxygen to produce an oxidation product and carbon monoxideas the reduction product of the carbon dioxide. Since carbon dioxide canonly oxidize carbon to carbon monoxide, the equation illustrating such acombustion or oxidation reaction is as follows:

To balance the above Equation 3 with respect to oxygen, the followingequation must apply:

Solving for m by merely transposing terms:

Then (m+a) the quantity of carbon monoxide which is measured, becomes:

By comparing Equation 6 with Equation 2, it will be seen that the valueof (m-t-a) is the same as the value of n wherein n is the oxygen demandexpressed in the number of atoms. In other words, the quantity of carbonmonoxide produced, in molecules (whether from the oxidation of carbon orthe reduction of carbon dioxide), is the same as the number of oxygenatoms that would be required for complete oxidation.

Consequently, by injecting the aqueous sample to be analyzed into aheated stream of carbon dioxide as in the above examples and passing thegases through an analyzer sensitized to determine carbon monoxide, oneis able to determine the total oxygen demand of the test sample. Ofcourse, if the sample also contains an oxidant, such oxidant detractsfrom the amount of oxygen that must be obtained from carbon dioxide.Nevertheless, the value measured represents the net total oxygen demand.

If desired, the net oxidizing capacity of a sample can be measured as afunction of the drop in carbon monoxide in the eflluent gas immedeiatelyafter injection of the sample. Such a measurement correlates with theoxidizing capacity of the sample analyzed and can be calibrated byreference to known standards in the 82111116 manner that peak heightsabove the base signal line can be calibrated to give TOD.

It should be realized that the two combustion conduits 22, may be atsomewhat different temperatures during operation, provided, of course,that the temperature of the first combustion must be high enough tovaporize the liquid part of the sample and to assure that none of thesample deposits on the conduit.

It is also realized that, with suitable valving, the mois tore-reducedeffluent could be re-run through the same combustion tube again, butsuch an arrangement is complicated to achieve without adverselyaffecting the accuracy and stability of the carbon monoxide detector.

What is claimed is:

1. A method for determining the oxygen demand of a material whichcomprises:

flowing a feed gas stream containing carbon dioxide as essentially thesole oxidant into an enclosed heating zone at a temperature of at least500' C. and containing a suitable catalyst bed, said catalyst bed beingetfective to promote the equilibration of carbon dioxide with oxidizablecomponents of the material to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe gas stream on the upstream side 1 l of the catalyst bed, removingmoisture from the gaseous product formed through the catalyst bed andunreacted feed gas from said heating zone, passing said moisture reducedgaseous product and unreacted feed gas again through an enclosed heatingzone at a temperature of at least 500 C. and containing a catalyst bedof the above mentioned type, and sweeping gaseous product from the lastmentioned heating zone and unreacted feed gas from the heating zone intoan analyzer for quantitatively indicating carbon monoxide.

2. A method as in claim 1 wherein said heating zones are maintained at atemperature within the range from 500 to 1000 C.

3. A method as in claim 1 wherein the flowing feed gas stream isestablished at a constant and predetermined flow rate.

4. A method as in claim 3 wherein the analyzer continuously monitors themixture of gaseous product and unreacted feed gas to produce anelectrical signal relative thereto and calibrating such signal todetermine the total oxygen demand of the material analyzed.

5. A method as in claim 3 wherein said catalyst beds each comprise anoble metal.

6. A method as in claim 3 wherein the material analyzed is an aqueousdispersion having organic components.

7. A method for determining the oxygen demand of a material whichcomprises:

incorporating a reducing agent as a gas into a flowing feed gas streamcontaining carbon dioxide as essentially the sole oxidant,

flowing the feed gas stream containing the reducing agent and carbondioxide into an enclosed heating zone at a temperature of at least 500C. and through a suitable catalyst bed disposed in said heating zone,said catalyst bed being effective to promote the equilibration of carbondioxide with oxidizable components of the material to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe gas stream on the upstream side of the catalyst bed,

drying the gases emanating from said heating zone,

passing said dried gases again through an enclosed heating zone at atemperature of at least 500 C. and containing a catalyst bed of theabove mentioned type, and

sweeping said gases from said last heating zone into an analyzer forquantitatively indicating carbon monoxide.

8. A method as in claim 7 wherein the flowing feed gas stream isestablished at a constant and predetermined flow rate and said heatingzone is maintained at a temperature within the range from 500 to 1000 C.

9. A method for determining the oxygen demand of a material whichcomprises:

flowing a feed gas stream containing carbon dioxide as essentially thesole oxidant into carbon monoxidegenerating means whereby a small amountof carbon monoxide relative to the carbon dioxide is introduced into thefeed gas,

flowing the feed gas stream into an enclosed heating zone at atemperature of at least 500 C. and through a catalyst bed disposed inthe heating zone, said catalyst bed being eflective to promote theequilibration of carbon dioxide with oxidizable components of thematerial to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe feed gas stream on the upstream side of the catalyst bed,

drying the gaseous mixture emanating from said heating zone, passingsaid dried gaseous product and eflluent in th o gh an enclosed h ting zne at a temperature of at least 500 C. and containing a catalyst bed ofthe above mentioned type, and sweeping the gaseous mixture into ananalyzer for quantitatively indicating the amount of carbon monoixde insaid gaseous mixture. 10. Apparatus in accordance with claim 9, whereinsaid means for circulating gas includes a reservoir of pressurized gas.

11. A method as in claim 9 wherein the flowing feed gas streamcontaining carbon dioxide as essentially the sole oxidant is establishedat a constant and predetermined flow rate from a uniform supply ofcarbon dioxide-containing gas and the carbon monoxide-generating meanscomprises a bed of carbon heated at a temperature in the range fromabout 300 to 600 C.

12. A method as in claim 11 wherein the material analyzed is an aqueousdispersion containing organic components.

13. A method as in claim 11 wherein the analyzer continuously monitorsthe carbon monoxide content of the gaseous mixture from the heating zoneand produces an electrical signal relative thereto.

14. A method as in claim 13 including the additional step of calibratingthe electrical signal indicating increased carbon monoxide, to determinethe total oxygen demand (TOD) of the material analyzed.

15. A method as in claim 13 including the additional step of calibratingthe electrical signal indicating decreased carbon monoxide, to determinethe oxidizing capacity of the sample analyzed.

16. Apparatus for determining the total oxygen demand of a material,comprising:

(a) first and second combustion conduits each including in inlet, andoutlet, a heating zone having a catalyst body of a material whichpromotes the equilibration of carbon dioxide with the oxidizablecomponents of the material analyzed at an elevated temperature, saidzone being disposed between said inlet and outlet,

(b) means for heating to and maintaining said heating zones atpredetermined operating temperatures,

(c) moisture removal means, said moisture removal means being disposedbetween the outlet of the first combustion conduit and the inlet of thesecond combustion conduit,

(d) carbon monoxide detection means having a fluid inlet, said fluidinlet of said detection means being coupled to the outlet of said secondcombustion conduit,

(e) means for circulating gas through said combustion conduits and saiddetection means, and

(f) means for introducing material to be analyzed to said heating zoneof said first combustion conduit.

17. Apparatus in accordance with claim 16, wherein said means forcirculating gas includes means for converting carbon dioxide to carbonmonoxide in small amounts, and said means for converting is disposedbetween said reservoir and the inlet of said first combustion conduit.

18. Apparatus in accordance with claim 16, wherein said carbon dioxidedetection means is an infrared analyzer which. produces an electricalsignal relative to the carbon monoxide content of a gas stream passingtherethrough.

References Cited UNITED STATES PATENTS 3,421,856 1/1969 Stenger et al.23230PC MORRIS O. WOLK, Primary Examiner E. A. KATZ, Assistant ExaminerUS. (:1. X.R. 23 232, 253, 254

